One of the things the First Look addressed only tangentially was the tamper. I thought I would hate the stubby-handled thing, but after a week of using this machine for my home (so, maybe a dozen shots in a day where I'm playing with beans; two-three on a normal day), I don't. The tamper storage system is clever, helps keep the work area clear, and the tamper itself is more than usable (again, in a domestic environment). If I were doing hundreds of shots a day, I'd probably move on to something that allows me to use the palm of my hand. But for a few shots here and a few shots there, finger pressure is enough. For me, at least.

Curious as to what led you to this conclusion? I'd like to hear your thoughts.

I've believed, at least until now, that the Scace II would be just the ticket to adjust an OPV since it diverts the (approximately) correct fraction of the pump output away from the OPV.

The flaw in adjusting an OPV under deadhead conditions is that the OPV is passing 100% of the pump output. But during the shot, only a portion (maybe 50%?) of the pump output goes through the OPV, and the rest goes through the puck. So the OPV is going to be open fractionally more under deadhead conditions than when actually brewing coffee.

The Scace II simulates the presence of the puck, reduces the flow through the OPV, and in my mind results in a more accurate OPV adjustment. Maybe there is a technical difference between "calibration" and "adjustment", but whatever you call it the bottom line is to set the machine up to simultaneously provide the correct flow and pressure at the puck.

The problem is that the Scace and the OPV are both doing the same thing, limiting the pressure, so you can't even see what pressure level the OPV is set at in that configuration. Under operating conditions, you want the OPV to limit the pressure when the grind is too fine, not when it's perfect. The best way to do that is by adjusting it with a blind.

I'll need to ponder that. I've always looked at this problem from the point of view of the pump's operating curve, and by observing that the OPV also has an operating curve of discharge versus pressure.

At a particular pressure, the vibe pump spits out a known volume per second (neglecting some secondary effects). For instance, when working against 9 bar, the Ulka E5's average operating curve says the output is approx. 260 ml/minute. But brewing a double only requires approx. 120 ml/minute (50 ml in 25 seconds).

So the ideal adjustment point in this example would be so that the OPV passes 140 ml/minute when the pressure at the puck is 9 bar. The pressure at the OPV when a blind filter is forcing the OPV to divert all of the pump discharge, i.e. 260 ml/minute, is a good first approximation of the correct adjustment. But it is not as accurate as making the adjustment when the correct flows are passing through the puck and the OPV.

All I need to know is that when the pressure at the puck measures 9 bar, the OPV is diverting just enough water (140 ml/minute in this scenario) to allow me to make a 25 second double. So in my view adjusting the OPV is not so much about limiting pressure (although it does this, too). It is more about getting the OPV to be open by exactly the right amount to divert the excess pump discharge away from the puck.

The OPV only sets a high limit on pressure, it doesn't know anything about flow rate. And the low limit is set by grind, dose, and tamp so the OPV isn't involved except in cases where the low limit is above the high limit, right?

The OPV only sets a high limit on pressure, it doesn't know anything about flow rate. And the low limit is set by grind, dose, and tamp so the OPV isn't involved except in cases where the low limit is above the high limit, right?

I don't think so. The OPV has to be involved at the low limit, too, or the excess pump discharge has nowhere to go.

Consider your model in a situation where the puck sets the low limit at 9 bar and the OPV is set (blind filter) at 11 bar. The pump feels 9 bar, outputs 260 ml/minute, and the OPV stays closed. One of two things is going to happen: either you get a 9 bar, 260 ml/minute gusher, or the puck somehow fights back and raises its resistance to a point where you get a normal extraction (120 ml/minute) at a pressure a little below 11 bar.

Unless there is some way of externally reducing the pump discharge (could be done electronically), the only way you're going to get both the right extraction rate and pressure is by having the OPV shunt an appropriate fraction of the pump discharge for all shots.

As an empirical matter, I find it easy to get a proper shot after setting the OPV to a max iof 9.5 bar with a blind and using proper grind and dose. If the grind is too loose, the pressure falls below 9 bar for all OPV settings 9 bar and above. That's what I mean by low limit.When the OPV is set to a high value, the measured pressure will tend to fall about half a bar during a shot. My theory is that that a proper puck will put up 9-10 bars of resistance by itself, and the OPV keeps that in the narrower range of 9 - 9.5 bar.

Say you put small resistor in series with the pump so that the pump's supply voltage is lowered by a few percent?

If the OPV were simply a pressure limiting device, no adjustment would be needed. The pump would do its thing until it had ramped up to 9.5 bar, then the OPV would snap open and maintain the pressure at 9.5 bar, right?

In practice, though, you'd see a tightening adjustment to the OPV would be needed to maintain the 9.5 bar blind filter reading. The tightening would be needed because the pump's output would be reduced (lower supply voltage).

In reality, the blind filter OPV adjustment point is based on passing 100% of the pump's discharge, so when the pump's discharge goes down the OPV needs to be adjusted accordingly or the pressure will go down.

Getting back to the original point, this illustrates why a device like a Scace II, which simulates both the flow and pressure targets for a shot, is effective.

The procedure of making a 0.5 bar allowance when setting the OPV is a useful approximation. It results in tightening the OPV a little bit past 9.0 bar so that it doesn't grab 100% of the pump discharge at brew pressure. But a device like the Scace II, which directly sets the correct flow conditions, eliminates the need to make that approximation.

I'm not suggesting that there's anything wrong with the empirical method you described. I'm just suggesting that the Scace II gets to the right OPV adjustment in a single step and doesn't rely on approximations.

I've been using the Breville ever since I bought in last October. It's performed remarkably well until this week. The OPV stayed at zero when trying to pull a shot. After some diagnosis on the phone with Breville, they offered to swap it for a brand new machine. The CSA was incredibly knowledgable about every aspect of the product. One of my hesitations about buying this machine was its durability. But this problem was apparently known and was related to it being one of the first units sold. I was told the problem was subsequently addressed by the company.

One other minor note I would offer to potential buyers. The front surface of the machine gets easily scratched..Apply a clear strip of packaging tape over it to avoid the problem. You have to realize that this machine is constructed quite differently from Italian machines. The Brevillle is built around a molded inner frame and is covered with stainless steel panels that are very thin and relatively soft.

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